1. Field of the Invention
The invention relates to an additive combination of an oil-soluble aliphatic copolymer having the property of a nucleator for wax crystallization e.g., ethylenevinyl acetate copolymer, with an oil-soluble derivative of an aromatic copolymer. This combination is particularly useful in distillate fuel oil having a final boiling point above about 370.degree. C., for the prevention of oil gelation and/or particularly controlling the size of wax crystals that form at low temperatures.
2. Description of the Prior Art
Various polymers, useful as middle distillate pour point depressants, prepared from ethylene have been described in the patent literature. These pour depressants include copolymers of ethylene and vinyl esters of lower fatty acids such as vinyl acetate (U.S. Pat. No. 3,048,479); copolymers of ethylene and alkyl acrylates (Canadian Patent 676,875); terpolymers of ethylene with vinyl esters and alkyl fumarates (U.S. Pat. Nos. 3,304,261 and 3,341,309); polymers of ethylene (British Pat. Nos. 848,777 and 993,744); chlorinated polyethylene (Belgian Pat. No. 707,371 and U.S. Pat. No. 3,337,313), etc.
Polymers having alkyl groups in the range of C.sub.6 to C.sub.18, such as homopolymers and copolymers of olefins, alkyl esters of unsaturated dicarboxylic acids (e.g., copolymers of dialkyl fumarate with vinyl acetate) and copolymers of olefins and said esters, are known in the art principally as lube oil pour depressants and/or V.I. improvers. For example, U.S. Pat. No. 2,379,728 teaches olefin polymers as lube pour depressants; U.S. Pat. No. 2,460,035 shows polyfumarates; U.S. Pat. No. 2,936,300 shows a copolymer of dialkyl fumarate and vinyl acetate; while U.S. Pat. No. 2,542,542 teaches copolymers of olefins, such as octadecene, with maleic anhydride esterified with alcohol, e.g., lauryl alcohol, in lubes and heating oils.
Synergistic pour point depressing combinations of various members of the above-noted two types of polymers in heavy fuels, e.g., residua and flash distillate fuels, which fuels contain relatively large amounts of waxes having on average 20 or more carbon atoms in the molecules, are taught in U.S. Pat. No. 3,726,653. The cold flow of middle distillate fuels is improved by the additive combination of 1000 to 3000 molecular weight (Mn) ethylene copolymer, such as ethylenevinyl acetate, and the polymer of a lauryl acrylic acid ester according to U.S. Pat. No. 3,275,427.
United Kingdom Specification No. 1,374,051 corresponding to U.S. Pat. No. 3,961,916 teaches that the low temperature flow characteristics of hydrocarbon middle distillates can be very satisfactorily controlled by the proper choice of a combination of a nucleating agent or wax growth stimulator and a wax crystal growth arrester.
U.S. Pat. No. 3,449,250 teaches that certain physical as well as some chemical properties of the heavier than gasoline liquid hydrocarbons such as fuel oils and lubricating oils, preferably those that have an initial boiling point of around 260.degree. C., are markedly improved by the addition of a minor amount of a copolymer which imparts stability and detergency to said liquid hydrocarbons. The copolymer has a molecular weight in the range of about 500 to 150,000 and is formed of substantially equimolar portions of maleic anhydride and alpha-olefins such as ethylene, propylene, isobutylene, or styrene. The carboxyl groups of the copolymers are esterified with an aliphatic alcohol to make the copolymer oil soluble and substantially all of the remainder of the carboxyl groups are imidized.